Cement raw meal used in the production of cement clinker, along with the fossil fuels used in the cement manufacturing process, both contain undesirable materials such as chlorine and sulfurs which vaporize in the temperatures encountered in a rotary kiln, are carried out of the kiln in the kiln exhaust gases and thereafter re-condense in the preheater region of the cement clinker plant. These materials build up over time and are harmful both to cement quality and the efficiency of the cement making process and hinder the “uptime” of the operation.
To reduce the buildup of such materials it has been known to utilize a cement bypass gas removal system, in which a portion of the kiln exhaust (i.e “off”) gases containing the volatile compounds is directed away from the preheater system. The exhaust gases enter a bypass conduit typically at either the lower region of the rotary kiln exhaust gas riser duct or the rotary kiln entry chamber. The optimum amount of exhaust gases directed off via such a bypass will typically begin at about 10% and will increase proportionately with the amount of undesired vaporized materials in the exhaust gas stream, which in turn is a function of the specific components of the raw materials. At cement plants that utilize raw materials that contain particularly large amounts of undesirable materials amounts as high as about 40-50%, and at times up to about 70%, by mass of the kiln exhaust gases are directed into the bypass conduit. Once in the bypass the kiln exhaust gases are typically cooled, such as by inserting external air into the bypass line or into a mixing chamber that is located within the bypass line. The cooling air quenches the kiln exit hot gases resulting in the condensation of the vaporous harmful substances contained in the bypass gas flow onto dust particles that are naturally entrained in the gases. The dust particles containing the condensed harmful substances are then separated from the cooled bypass gas flow in a dust separator.
A further pressure on cement producers in the United States is to reduce nitrogen oxide (NOx) emissions that are a byproduct of the cement manufacturing process. Cement manufacturing conditions are particularly favorable for formation of NOx because of the high process temperatures and oxidizing atmospheres involved. Since NOx is an ozone-forming precursor, uncontrolled or under controlled NOx emissions from cement manufacturing are widely recognized as exacerbating ozone related problems.
Federal New Source Performance Standards mandate NOx emissions limits in the main stack of cement plants to 1.5 lb NOx/ton of cement clinker. To meet these standards cement manufacturers primarily utilize a Selective Noncatalytic Reduction (SNCR) system which basically entails the injection of ammonia in the form of ammonia water or urea in the flue-gas at area in the cement making process where such flue gas is at a suitable temperature. An ammonia solution (appx. 20%) is the reagent that has been most often used for cement kilns, with such a solution being particularly effective for preheater/precalciner cement kiln applications. Urea is also known to be a suitable reagent. For such applications, it is taught to locate the NOx system at a point in the cement manufacturing process at which the temperature of the gas stream is within the temperature window appropriate for an SNCR system, and in particular for reagent injection. Typical locations at which the temperatures are suitable are at the precalciner exit and in the lower section of the cyclone preheater tower.
Federal NOx standards become increasingly difficult to achieve economically as the amount of exhaust gases drawn off via cement kiln bypass systems increases. Therefore, in the prior art bypass gases cleansed of volatiles are redirected to the precalciner and thereafter to the NOx reduction system incorporated in the cement manufacturing process. However, redirecting bypass gases in such a manner has been shown to result in significant increases in fuel consumption, thus making the process economically unfeasible. Taking the alternative route of directing bypass gases that are cleaned of undesirable materials to the main stack or other areas of the plant will result in higher NOx rates which may prevent NOx emission standards from being met.
It would be desirable, therefore, to have a method to reduce undesirable materials from kiln exhaust gases while achieving suitable NOx emission rates that does not significantly increase fuel consumption.